1. Field of the Invention
The present invention relates to the generation of a clock signal in an integrated circuit and, more particularly but not exclusively, to contactless tags, such as Radio-Frequency Identification tags (RFID).
2. Description of the Related Art
RFID tags generally comprise send and receive circuits for sending and receiving modulated radioelectric signals to exchange data with a reader, an electrical supply circuit to generate a supply voltage of the integrated circuit using the electromagnetic field generated by the reader, a processing unit, and a non-volatile memory, of EEPROM-type for example.
The send and receive circuits of the tag use a clock signal, the frequency of which must vary as little as possible to be able to decode the data received and to send signals capable of being decoded by a receive unit of a reader.
Furthermore, this clock signal must be synchronized with that of the reader. For this purpose, the reader sends the tag a reference signal, which the tag uses to adjust the frequency of its clock signal. To generate a clock signal having a constant frequency, a local oscillator is generally used that produces a reference frequency signal from which the clock signal is generated.
However, it is difficult to produce such a reference frequency in an integrated circuit such as a contactless tag. Indeed, oscillators are sensitive to their environment and to temperature variations in particular. Furthermore, a contactless tag can be subjected to very variable environments. Moreover, the internal temperature of a tag can vary significantly during a communication session with a reader. The result is that the reference frequency generated by the local oscillator can also vary during a communication session.
Furthermore, the reference signal enabling the tag to determine its clock frequency is generally only transmitted once, at the start of a communication session with a reader. The result is that the tag cannot re-adjust its clock frequency several times during a session.